RV Builder's Hotline: CG and the RV-8


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CG and the RV-8
by Terry Lutz

. Terry Lutz and his RV-8

I have done first flights and some test work on five RVs now, and three of them were RV-8s. Since I am a test pilot for a large manufacturer, I figure I have some of the tools to do a good evaluation of the airplanes.

The first one, N43LF, is a 180 hp airplane with a fixed-pitch prop. It was built fairly light, and weighed in about 1085 lbs empty. The Odyssey battery was mounted on the firewall.

One day, as I was practicing landings in light winds with about 1/4 fuel in each tank, I flew my normal pattern and established a power-off approach at 70 mph. I slowed into the flare and because of the light weight and forward cg, I found myself at full back stick and still 1-2 feet in the air.

I was in a 3-point attitude when I noticed I was at full back stick, and just about the time I pushed the throttle up, the nose began to pitch down and I touched briefly on the main wheels as I began the go-around. It was an uncomfortable feeling. The airplane was not at the stall, as you normally would be in a three-point landing, and I could not keep the tail down, as you normally could in a three point landing. I came back around and landed, intentionally keeping the speed a little higher.

One thing we can thank Van for is an airplane that flies very slowly with full flaps in the flare. But at forward cg and light weight, you can fly slow enough that you run out of elevator control. It is not a good feeling.

So I bought two 10-pound bags of cat litter, bound them up with a lot of duct tape so they wouldn't leak, and went out to do a little testing. The airplane should fly in trim with hands off and maintain final approach speed, and still have suitable elevator margin to overcome ground effect, and enough elevator for flare and landing. So I made sure the airplane was at light weight with about 1/4 fuel in each tank, and tested the airplane in three configurations: 2 bags of litter, one bag of litter, and no bags of litter in the aft baggage compartment.

I set up the test by slowing to approach speed with full flaps, applying full nose-up trim, and establishing an idle power glide. I noted the stabilized speed in each case. This required some planning so I could stay close to the airport, quickly establish the desired flight condition, then quickly land and change the ballast in the aft baggage compartment without getting short on fuel. It must have been a bit amusing to watching me land, taxi clear of the runway, remove a duct-tape-wrapped package, and take off again.

I put the bag just off the taxiway so it wouldn't be in anyone's way. The airport guys saw this and were just about to call the local sheriff when I finished the test and finally came in for fuel. They thought I was dropping off bags of marijuana, or some other sort of contraband!

Those quick tests showed that at light weight, I could not trim hands off at approach speed with full flaps and power off without two 10 lb bags of cat litter in the aft baggage.

That meant that with less than two 10 lb bags in the back, I would have to use a little back stick in addition to full nose up trim, to stabilize at approach speed. This small amount of elevator took away from the elevator I needed to safely flare and land at light weight. I recommended to the owner that when flying solo, he should always keep 20 lbs of ballast in the aft baggage compartment.

When I built my own RV-8, I planned in the beginning to use a constant-speed propeller, which puts the CG fairly far forward. I put the Odyssey battery and strobe flasher behind the rear seat, and did what I could to minimize weight up front. Even with those considerations, I find the need to carry the same 20 lbs of ballast when flying solo. This gives reasonable stick forces for aerobatics (I would prefer them to be just a bit lighter, but not as light as with two people), and I still have enough elevator to flare and land at light weight.

As an additional note, if you fly final carrying a small amount of power (pretty normal with a constant speed prop), it can give the impression that there is plenty of elevator for the flare. But with power off, more elevator is needed and you may find yourself on the aft stick stop and still in the air.

Reader comments:

Dan Miller, (RV-8), Battle Ground, WA.

I completely agree with Terry's assessment of RV-8 CG issues. My RV-8 has an IO-360 M1, Whirlwind 200RV prop, and battery and ELT behind the rear baggage compartment. When flying solo, I always carry a 20+ lb bag of tools strapped to the rear baggage shelf. This allows me to trim for pattern speed, gives me plenty of elevator left for flaring and landing 3-point, and helps lighten elevator forces for aerobatics.

An RV-8 with an angle-valve IO-360 and a Hartzell prop weighs about 58 lbs more on the nose than my configuration and would require even more weight in the rear baggage compartment to be able to flare at slow landing speeds.

Nic hart (RV-3) UK

Terry has it about right. The conundrum with the RV-8 is the center of gravity, which is a real achilles heel. Built nose heavy, it requires a ton of ballast in the back flying solo to achieve reasonable control harmony, (the ballast needs to be taken in and out depending on your mission) and without ballast as Terry says, elevator authority has a tendency to run out at critical times during short-field landings.Built tail heavy, there is naturally a limit to passenger and rear baggage.

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This story appeared in the February 14, 2009 issue of the RV Builder's Hotline.